Optical fibers are increasingly being used in the field of communications not only to transmit information, but also as repeaters to amplify the encoded optical signals. For example, amplifiers comprising erbium (Er) doped fibers are now commercially available from BT&D Technologies. See also, Mears, et al., "Low-noise erbium-doped fiber amplifier operating at 1.54 .mu.m," Electron. Lett., Vol. 23, pp. 1026 et seq. (1987); and Suzuki, et al., "Subpicosecond soliton amplification and transmission using Er.sup.3+ -doped fibers pumped by InGaAsP laser diodes," Optics Letters, Vol. 14, No. 16, pp. 865 et seq. (1989). The advent of such Er amplifiers presents the possibility of long-distance, high-speed soliton based communication. There is thus a need for a simple, inexpensive, efficient, and preferably all fiber source of solitons at a wavelength compatible with Er amplifiers and a repetition rate suitable for the new high speed communications systems.
Prior to the present invention, various types of short and soliton pulse lasers were known. For example, Mollenauer, et al., "The soliton laser," Optics Letters, Vol. 9, No. pp. 13-15 (January 1984), describes a synchronously pumped, mode-locked color-center laser coupled through a beam splitter to a feedback loop incorporating a length of single-mode, polarization-preserving fiber. As the laser action builds up from noise, the initially broad pulses are considerably narrowed by passage through the fiber. The narrowed pulses, reinjected into the main cavity, force the laser to produce narrower pulses This process builds upon itself until the pulses in the fiber become solitons, that is, until the pulses have substantially the same shape following their double passage through the fiber as they had upon entry. U.S. Pat. No. 4,635,263 to Mollenauer discloses the use of media other than color center lasers, such as semiconductors or the fiber itself to provide the necessary gain. In each case, such a soliton laser takes the form of a single loop ring laser having a short gain fiber coupled to a longer pulse shaping fiber. U.S. Pat. No. 4,835,778 to Kafka et al. discloses a subpicosecond fiber laser comprising an Er doped gain fiber joined to a pulse shaping fiber section in a resonant cavity in either a linear or single closed loop configuration. Initially formed pulses recirculate many times in the resonator to shorten the pulses until a steady state condition is reached. Stankov, "A Mirror with an Intensity-Dependent Reflection Coefficient," Applied Physics B, Vol. 45, pp. 191-195 (1988), describes the use of one type of nonlinear mirror with power dependent reflection to mode-lock a Nd:YAG laser and thereby obtain short light pulses of subpicosecond duration. In the Stankov device, an intense light beam at frequency .omega. generates a second harmonic beam in a nonlinear crystal. The total second harmonic at 2.omega. and part of the fundamental beam are reflected by a dichroic mirror back through a phase-adjusting glass plate to provide the necessary phase relation between the two reflected light waves, and then back through the nonlinear crystal. In the second pass through the crystal, partial reconversion of the second harmonic into the fundamental wavelength takes place. The degree of conversion and reconversion is dependent on the intensity of the incident beam. U.S. Pat. No. 4,904,041 to Izadpanah discloses another example of a short optical pulse generator in which the optical radiation from a driven laser diode is coupled to an external cavity including a looped directional coupler which causes the laser to mode-lock and produce an output stream of very short high repetition pulses.
It was also known that optical soliton switching could be achieved in an all-fiber nonlinear loop mirror or Sagnac (anti-resonant ring) interferometer. See Doran, et al., "Nonlinear-optical loop mirror," Optics Letters, Vol. 13, No. 1, pp. 56-58 (January 1988); and Blow, et al., "Experimental demonstration of optical soliton switching in an all-fiber nonlinear Sagnac interferometer," Optics Letters, Vol. 14, No. 14, pp. 754-756 (July 1989) One embodiment of the present invention also employs a nonlinear amplifying mirror (NALM) developed by Fermann et al. for use in optical switching applications, which is described in "Nonlinear amplifying loop mirror," Optics Letters, Vol. 15, No. 13, pp. 752-754 (July 1990).